1. Field of the Invention
This invention relates generally to digital communications, and more particularly, to digital coding (or compression) of speech and/or audio signals.
2. Related Art
In the field of speech coding, the most popular encoding method is predictive coding. Most of the popular predictive speech coding schemes, such as Multi-Pulse Linear Predictive Coding (MPLPC) and Code-Excited Linear Prediction (CELP), use two kinds of prediction. The first kind, called short-term prediction, exploits the correlation between adjacent speech samples. The second kind, called long-term prediction, exploits the correlation between speech samples at a much greater distance. Voiced speech signal waveforms are nearly periodic if examined in a local scale of 20 to 30 ms. The period of such a locally periodic speech waveform is called the pitch period. When the speech waveform is nearly periodic, each speech sample is fairly predictable from speech samples roughly one pitch period earlier. The long-term prediction in most predictive speech coding systems exploits such pitch periodicity. Obtaining an accurate estimate of the pitch period at each update instant is often critical to the performance of the long-term predictor and the overall predictive coding system.
A straightforward prior-art approach for extracting the pitch period is to identify the time lag corresponding to the largest correlation or normalized correlation values for time lags in the target pitch period range. However, the resulting computational complexity can be quite high. Furthermore, a common problem is the estimated pitch period produced this way is often an integer multiple of the true pitch period.
A common way to combat the complexity issue is to decimate the speech signal, and then do the correlation peak-picking in the decimated signal domain. However, the reduced time resolution and audio bandwidth of the decimated signal can sometimes cause problems in pitch extraction.
A common way to combat the multiple-pitch problem is to buffer more pitch period estimates at “future” update instants, and then attempt to smooth out multiple pitch period by the so-called “backward tracking”. However, this increases the signal delay through the system.